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In that story, after memories are sufficiently embedded, removing them leaves so much empty space it is impossible for the mind to heal itself around the missing pieces. They end up turning the guy into someone else entirely to keep him alive.

I wonder if Mycroft abandoned the project because he thought of that possibility? Maybe one of the Next family is already walking around with a split personality??

I'd say a mnemonomorph like Aornis has the subtlety and skill to actually heal (as well as cause) mindworms and other complex memories. Rather than just ripping out a chunk of memory and everything it's connected to, they'd be able to go through and trace the specific memory, extracting it without harming the other memories. So if there were another mnemonomorph out there, he or she ought to be able to heal Thursday's mindworm (if skilled enough). On the other hand, memory erasure technology might never be as subtle as that.

Oh, and the change in first person caught me too - in fact, at the start of the next chapter I grasped that we were now hearing Thursday1-4's narrative, but I assumed the switch had been made between chapters and it wasn't until the real Thursday said, "I would have done the same" that I realized it had been the double ever since the fight!